Superdirective arrays, i.e., antenna arrays with element spacings significantly less than .lambda..sub.o /2, where .lambda..sub.o is the resonant wavelength, have, in the past, not been practical to implement because the radiation resistance is generally small compared with the ohmic resistance. Also, very high currents are required to flow in the antenna elements, to achieve any significant amount of radiation.
Because of the essentiality of understanding the technical terms relating to directive antennas, selected pertinent terms will now be defined hereinbelow.
Radiation resistance is obtained by dividing the total radiated power of the antenna by the square of the effective antenna current measured at the point where power is supplied to the antenna.
Ohmic resistance is the opposition that a device or material offers to the flow of direct current, measured in ohms, kilohms, or megohms.
A lobe is one of the three-dimensional portions of the radiation pattern of a directional antenna. The direction of maximum radiation coincides with the axis of the major lobe. All other lobes in the patterns are called minor lobes.
Antenna power gain is a transmitting antenna rating equal to the square of the antenna gain, expressed in decibels.
Decreasing the value of the ohmic resistance would solve one of the major problems associated with achieving maximum radiation efficiency. Then the radiation resistance compared to the ohmic resistance would be at a value where superdirective arrays can become a practical reality as further projected below.
Thus, solving the problems relating to high ohmic resistance can result in very compact antennas. These antennas having high gain, low side lobes, and high directivity also have narrow beam widths. By way of example, a broadside array with 25 elements separated by .lambda..sub.o /24, with length of .lambda..sub.o, designed as a Chebyshev array with side lobes of 20 dB down would have a beam width of 13.degree..
A Chebyshev array, such as above, provides the design criteria for establishing the current distribution in the array elements which will produce a minimum beamwidth for a given sidelobe level. As one attempts to get enhanced directivity with element spacings &lt;(.lambda./2) for a given overall antenna dimension L, a large value of Q is the result, where Q is defined: ##EQU1##
The bandwidth of the antenna is ##EQU2##
High gain or supergaining relative to an antenna results in a decreased bandwidth of the antenna. However, the trend in radar, missile guidance and communications links is toward wide bandwidth signals for: (1) enhanced channel capacity, (2) countermeasures hardening, or (3) improved detection and discrimination capability. A superconducting, superdirective antenna array with a very high radiation efficiency is highly desirable since it would have an extremely small bandwidth. Such an array as a high gain radiator of high power microwave or VHF (very high frequency) source which has high directivity is extremely important for weapons application.
Therefore, an object of this invention is to provide a superconducting, superdirective antenna array.
Another object of this invention is to provide a superconducting, superdirective antenna array with a very small bandwidth signal for communications which will be almost impossible to intercept with ordinary receivers.
A further object of this invention is to provide a superconducting, superdirective antenna array whereby the superdirective signal may be used as an active but covert radar, if doppler or range-doppler information only is required.